refactor(store): use null instead of undefined for optional state properties

* remove setConnectionNodeId action

src/types/node.ts

@@ -1,6 +1,7 @@
 import { CSSProperties } from 'vue'
 import { XYPosition, Position, SnapGrid, Element, XYZPosition, Dimensions } from './flow'
-import { DefaultNodeTypes, HandleElement, ValidConnectionFunc } from './components'
+import { DefaultNodeTypes } from './components'
+import { HandleElement, ValidConnectionFunc } from './handle'
 
 export type CoordinateExtent = [[number, number], [number, number]]